1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to an image pickup apparatus configured such that a pixel includes a signal holding unit.
2. Description of the Related Art
It is known to configure an image pickup apparatus such that each pixel has an amplifying element, as is known as an in-pixel amplification type. In the image pickup apparatus of the in-pixel amplification type, each pixel is capable of holding a signal in a photoelectric conversion unit and at an input node of an amplifying element. For use in such an image pickup apparatus of an in-pixel amplification type, a global electronic shutter technique has been developed which allows it to obtain an equal exposure period over an entire image sensing plane. A plurality of configurations are known to achieve a global electronic shutter. In one configuration, a signal holding unit is disposed, separately from a photoelectric conversion unit and an amplifying element, in an electric path between the photoelectric conversion unit and an input node of the amplifying element. It is also known to dispose a plurality of signal holding units between a photoelectric conversion unit and an input node of an amplifying element (see, for example, Japanese Patent Laid-Open No. 2009-296674 and Japanese Patent Laid-Open No. 2011-217315). However, in the configuration in which a plurality of signal holding units are disposed in an electric path between an output node of a photoelectric conversion unit and an input node of an amplifying element, no much consideration has been given to the efficiency of transferring a signal charge. In an image pickup apparatus including a first signal holding unit and a second signal holding unit disposed such that the first signal holding unit is located close to a photoelectric conversion unit and the second signal holding unit is located between the first signal holding unit and an amplifying element, a signal charge generated in the photoelectric conversion unit reaches the input node of the amplifying element after travelling through the first signal holding unit and the second signal holding unit. When the signal charge travels through an electric path, if a proper potential is not formed in the electric path, part or all of the signal charge may stop somewhere in the electric path, which may result in a problem.